Fix rounding with digits/sigdigits

[sostock]

Fixes #307

This PR makes round(u::Units, x::Quantity, digits/sigdigits=...) return a quantity based on a floating-point number (as does rounding with digits/sigdigits for unitless numbers).

[codecov-io]

Codecov Report

Merging #308 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #308   +/-   ##
=======================================
  Coverage   81.39%   81.39%           
=======================================
  Files          15       15           
  Lines        1129     1129           
=======================================
  Hits          919      919           
  Misses        210      210

Impacted Files	Coverage Δ
src/quantities.jl	93.04% <100%> (ø)	⬆️
src/pkgdefaults.jl	30.61% <0%> (ø)	⬆️

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[ajkeller34]

Thanks!